Brain research. Developmental brain research最新文献

Embryonic ventral midbrains from GFAP-lacZ transgenic mice were xenografted into the dopamine-depleted striata of adult rats. This transgenic line harbors a nuclear-targeted bacterial beta-galactosidase (beta-gal) reporter gene under transcriptional control of the human glial fibrillary acidic protein (GFAP) promoter sequence. Five weeks post-transplantation, graft-derived astrocytes and dopaminergic neurons were visualized by dual immunocytochemistry for beta-gal and tyrosine hydroxylase (TH), respectively. This report describes the advantages associated with the use of GFAP-lacZ transgenic mice to study astrocyte fate in embryonic neural grafts.

The present studies were designed to study the interrelationships between GABAergic, serotoninergic and excitatory amino acids systems (EAAs) in the control of gonadotropin secretion in prepubertal female rats. For this purpose we determined the effects of N-methyl-D-aspartate (NMDA), an exogenous agonist of EAAs receptors, on LH and FSH secretion in 16-day-old female rats in which the GABA-A and GABA-B receptors were blocked by bicuculline and baclofen or serotonin (5-HT) depleted by p-choloroamphetamine (PCA). In addition the effects of the GABAergic and serotoninergic systems on LH and FSH secretion were evaluated in animals treated with dibenzocycloalkenimine (diocilpine MK-801), an antagonist of NMDA neurotransmission. While muscimol, a GABA-A agonist, induced a significant increase in LH and FSH levels (P < 0.01), baclofen, a GABA-B agonist, had an inhibitory effect on these hormones (P < 0.01). MK 801, a NMDA receptor antagonist, not only suppressed the stimulatory effect of NMDA on LH and FSH but also blocked the stimulatory effect of muscimol without modifying the inhibitory action of baclofen on both gonadotropins. Bicuculline, a GABA-A receptor antagonist, did not modify the release effect of NMDA on LH and FSH. 5-HTP, a precursor of 5-HT that increases the levels of this neurotransmitter in the central nervous system significantly increased (P < 0.01) the plasma levels of LH and FSH, and this effect was blocked by the NMDA receptor antagonist MK-801. We conclude that the stimulatory effects of GABAergic and serotoninergic systems in prepubertal female rats are connected with the activation of EAA neurotransmission, while the stimulatory effects of NMDA appear to be independent of serotoninergic and GABAergic actions on LH and FSH secretion. Since both GABA and serotonin systems change their effects on LH and FSH during sexual maturation from a stimulatory action in prepubertal to an inhibitory action in adult rats and since NMDA neurotransmission has a stimulatory effect on gonadotropin secretion both in prepubertal and adult rats, it is clear that the interrelationships between GABAergic and serotoninergic systems with EAAs in the gonadotropin control are different in prepubertal and in adult rats.

We have employed a collagen-gel co-culture system to evaluate the influence of the hindbrain floor-plate on initial axon outgrowth from the cochlear nucleus. After 2 days in vitro, cochlear nucleus explants exhibited directed outgrowth towards co-cultured floor plate explants. Comparisons with co-cultures of cochlear nucleus/forebrain or cochlear nucleus/midbrain explants or with cochlear nucleus explants cultured alone suggest that the floor-plate has a specific chemoattractant effect on the outgrowth of the pioneer fibers of the hindbrain auditory commissure. Fiber outgrowth was not directed towards a recombinant source of the chemoattractant molecule netrin suggesting that floor plate directed outgrowth in the cochlear nucleus is not solely dependent on netrin. In the present report, we present evidence for the first time that the floor-plate is a chemotropic source in pathfinding of second-order auditory fibers from the cochlear nucleus in the hindbrain.

In the mouse nasopalate papilla and in the trenches of the foliate and vallate papillae, taste buds accumulated primarily during the first 2 weeks after birth. Null mutation for brain-derived neurotrophic factor caused extensive death of embryonic taste neurons, with the secondary outcome that most taste buds failed to form. However not all taste neurons died; functional redundancy rescued a variable number. The primary research objective was to identify the likely site of the taste neuron rescue factor that substituted for BDNF. In this quest taste bud abundance served as a useful gauge of taste neuron abundance. The proportion of taste buds that developed was variable and uncorrelated among the nasopalate, vallate, and foliate gustatory papillae within each bdnf null mutant mouse. Thus, in spite of shared IXth nerve innervation, the vallate and foliate papillae independently varied in residual gustatory innervation. This variation rules against the rescue of gustatory neurons by system-wide factors or by factors acting on the IXth ganglion or nerve trunk. Therefore it is likely that surviving BDNF-deprived taste neurons were stochastically rescued by a redundant neurotrophic factor at the level of the local gustatory epithelium. These findings broaden the classic expectation that target tissue supplies only a single neurotrophic factor that can sustain sensory (taste) neurons.

Paired-pulse facilitation (PPF) of CA3-CA1 excitatory postsynaptic potentials (EPSP) was compared in hippocampal slices from juvenile (postnatal day (P) 15-21) and young adult rats (P28-P35) following application of adenosine. Relative to juveniles, young adults expressed an increase in baseline synaptic strength that was accompanied by a decrease in PPF suggesting a developmental increase in transmitter release. While adenosine depressed the EPSP slope to a similar extent in juveniles and young adults, PPF increased during adenosine application only for young adults. The differential effect of adenosine on PPF was not due to differences in receptor function or in extracellular ligand levels, since the A1 antagonist cyclopentyltheophylline (CPT) did not differentially affect PPF across age. Adenosine could increase PPF in juvenile slices under conditions of enhanced transmitter release, through an increase in the bath Ca2+ concentration, or addition of forskolin to the bath. These data indicate that the ability to modify synaptic transmission through presynaptic adenosine A1 receptors increases across postnatal development with the maturation of release mechanisms.

In the present study the neuroprotective effect of mild hypothermia (decrease of temperature from 37degreesC to 33degreesC) during and after transient ischemia in brain tissue at different stages of development was tested in vitro by measuring energy metabolism, glutamate release and protein biosynthesis rate (PSR) in hippocampal slices. Slices were taken from immature (E40) and mature (E60) guinea pig fetuses and adult guinea pigs. The slices were exposed to ischemia-like conditions (oxygen/glucose deprivation, OGD) for periods of between 10 to 40 min followed by a 2-h or 12-h recovery phase. During OGD, mild hypothermia slowed down the depletion of energy stores only in slices from immature fetuses, but had no effect on slices prepared from mature fetuses and adult animals. Hypothermia also reduced glutamate release significantly during oxygen/glucose deprivation. Lowering temperature to 33degreesC had no effect on energy metabolism and only a minor effect on PSR of slices from mature fetuses and adult animals subjected to 2 h of recovery. However, 12 h after OGD PSR was markedly improved by mild hypothermia in slices from mature animals and in slices from adults that had been exposed to OGD for only 20 or 30 min. The inhibition of PSR was more severe in the slices from adults than in those from mature fetuses subjected to the same duration of OGD. Age- and temperature-related differences in glutamate release during OGD did not fully agree with corresponding disparities in the values for PSR obtained 12 h after OGD. These results indicate that the neuroprotective effect of mild hypothermia was not mediated by a temperature-dependent retardation of the depletion of energy stores during OGD. Age-related disparities in the vulnerability of the brain to ischemia and the neuroprotective efficiency of mild hypothermia appear to be only partially reflected by the varying levels of glutamate release during ischemia but best reflected by the extent of PSR inhibition. It is concluded that mild hypothermia may be a suitable therapeutical intervention for the suppression of hypoxic-ischemic cell damage during birth.

Cultured explants obtained from the dentate gyrus of rat embryos (embryonic day 19-20) were used to investigate synapse formation and morphological differentiation of neuron types in the absence of extrinsic afferents. Synaptogenesis was studied by whole-cell recordings of postsynaptic currents and by ultrastructural analysis. Neurons were visualized using Lucifer Yellow filling or staining with DiI. In short-term (3-5 days) cultured explants postsynaptic currents were rarely evoked by extracellular stimulation and synapses were almost completely absent at the ultrastructural level. After 6-10 days in vitro, the incidence of evoking postsynaptic currents mediated by glutamate and GABAA receptors was strongly increased. At the ultrastructural level, the density of synapses increased more than 20-fold. These results demonstrate de novo formation of synapses in cultured embryonic dentate gyrus explants. Neuron types could be discriminated by their dendritic arborizations and by their electrophysiological properties. After 6-10 days in vitro, mossy-like cells exhibited 3-4 primary dendrites branching in a characteristic pattern and showed moderate spike-frequency adaptation. Application of serotonin (5-HT) to cultured explants elicited GABAA-receptor-mediated postsynaptic currents in mossy-like cells, indicating synaptic GABA release from local interneurons. Comparison to 5-HT evoked GABA release in mossy cells in age-matched, acute slices revealed only slight quantitative differences. In contrast to mossy cells, granule cells showing several primary dendrites originating at one cell pole were almost completely absent in cultured explants, suggesting an involvement of extrinsic afferents in the differentiation of granule cells.

Previous studies of the nicotinic acetylcholine receptor (nAChR) subunits in adult mammalian and avian brains have demonstrated a spatially restricted distribution of these subunits; little, however, is known about the nAChR subunit developmental distribution. The present study demonstrated a transient pattern of distribution of the neuronal nAChR subunit, alpha7, in the developing chick cerebellum by using immunohistochemical techniques. This transient distribution may suggest a critical period for the development of the cholinergic system in the cerebellum.